Process for obtaining a low silicon aluminium alloy part

ABSTRACT

The part made of low-silicon aluminum alloy contains magnesium, copper, manganese, titanium, and strontium. Said part is obtained by a method that consists in:
         casting said alloy in a mold so as to obtain the part;   after the casting, demolding the part constituting a preform that is still hot;   cooling said preform and then subjecting it to an operation suitable for reheating it to a temperature lying in the range 470° C. to 550° C.;   positioning said part between two shells of a die that defines a cavity of dimensions substantially equal to but less than the dimensions of the cavity of the mold; and   strongly pressing the two shells together to exert on the part disposed between said shells a combined pressing and surface kneading effect.

The invention relates to the technical field of foundry work or casting, for manufacturing aluminum parts, in particular in the automobile and aviation sectors, and more generally in all types of industry.

Many alloys exists that are said to be “low-silicon” alloys. Such alloys have high mechanical characteristics after T6 heat treatment (Rp_(0.2) of 300 MPa; A % of 8%). They are grouped together in the 6000 (Al—Mg—Si) series in the classification of aluminum alloys. The most well known are the 6082, 6061, and 6151. Numerous compositions also exist with contents similar to the standardized alloys, among which mention can be made, for example, of Document EP 0 987 344.

The above-mentioned alloys have been developed for obtaining semi-finished products (billets or ingots for forging or rolling) designed to be transformed during hot or cold operations with high deformation rates (>50%).

In addition, the geometrical shapes of such semi-finished products are simple (bar, rod, or ingot), thereby making it possible to solidify such alloys with defects being minimized, by using methods having high solidification speeds. Such geometrical shapes and such methods result, using techniques that are currently mastered, to semi-finished products that are exempt from defects, such defects being, for example: shrink holes, cracks, macro-segregations, and macro-precipitations (formation of precipitates that are too coarse, >100 μm).

Based on that state of the art, the problem posed that the invention proposes to solve is to make it possible to obtain parts that satisfy high safety and quality standards, and that can be of complex shapes.

To solve this problem, the invention provides a method of manufacturing a part made of low-silicon aluminum alloy of the 6000 type.

More particularly, the invention provides a method of obtaining a part made of low-silicon aluminum alloy, containing silicon at a content lying in the range 0.5% to 3%, magnesium at a content lying in the range 0.65% to 1%, copper at a content lying in the range 0.20% to 0.40%, manganese at a content lying in the range 0.15% to 0.25%, titanium at a content lying in the range 0.10% to 0.20%, and strontium at a content lying in the range 0 ppm to 120 ppm, said method including:

-   -   casting said alloy in a mold so as to obtain the part;     -   after the casting, demolding the part constituting a preform         that is still hot;     -   cooling said preform and then subjecting it to an operation         suitable for reheating it to a temperature lying in the range         470° C. to 550° C.;     -   positioning said part between two shells of a die that defines a         cavity of dimensions substantially equal to but less than the         dimensions of the cavity of the mold; and     -   strongly pressing the two shells together to exert on the part         disposed between said shells a combined effect of pressing and         surface kneading.

The present invention also provides:

-   -   implementing the above method in the automobile sector or in the         aviation sector;     -   use of a part obtained by the above-mentioned method in the         automobile sector; and     -   use of the alloy in the above-mentioned method in the aviation         sector.

In an implementation of the method, after the preform has been cooled, it is reheated by being placed in a tunnel furnace.

As a result of these characteristics, the casting operation followed by the forging of the preform in one step do not have the same parameters as regards temperatures, solidification speed, rate of deformation, and forge temperature as the methods in the state of the prior art.

The alloy claimed satisfies these constraints and makes it possible to obtain parts of satisfactory quality, in particular if the parts have to satisfy safety obligations (suspension system parts=safety parts).

Among such constraints, the following may be mentioned by way of example:

-   -   the geometrical shape of the preform, unlike bars or ingots,         includes, as of being designed, the rough outlines of the         functional zones of the part, and can therefore have a complex         geometrical shape including ribs or variations in section         leading to isolated masses of liquid metal. Such isolated masses         can be “tolerated” by increasing the silicon content (grade         AS7G03, standard casting alloy). A reduction in that content         makes the alloy more sensitive during solidification and leads         to shrink-hole (porosity) defects that are greater in number and         in volume.     -   the solidification range, which is the range defined from the         liquidus temperature to the eutectic temperature of the alloy in         question. For a strontium-modified AS7G03 type alloy, this range         is about 50° C. (from 611° C. to 562° C.). For a low-silicon         6000 type alloy, it is about 90° C. (from 655° C. to 562° C.)         with the precipitation of the macroscopic Mg₂Si (or silicon) as         a pseudo eutectic line. A wide solidification range leads to a         semi-solid zone that extends further through the part, so that         it becomes more difficult to direct the solidification edge to         reduce defects as is done conventionally and almost naturally         with an AS7G03 alloy.     -   AS7G03 has almost zero sensitivity to cracking due to the large         quantity of eutectic that is able to fill in cracks that appear         during shrinking while solidification is taking place. This does         not apply for a low-silicon alloy, which has very little         eutectic, thereby resulting in high sensitivity to cracking and         requiring the composition to be adapted and the solidification         temperature gradients to be controlled.

It is also necessary to adjust the chemical composition so as to obtain a better compromise or trade-off between the parameters for casting, forging, and heat treatment and the desired mechanical characteristics for the finished parts. To this end, each of the elements of the alloy, its content, and the effects resulting in that value having been chosen are given in detail below:

The silicon content lies in the range 0.5% to 3%. A silicon content less than 1% results in the highest yield strengths and elongations. However, it is the content for which the alloy is the most sensitive to cracking and has the lowest castability or fluidity. It is therefore necessary to be able to adapt the silicon content as a function of the geometrical shape of the part. Complex geometrical shapes require a higher content so as to reduce this sensitivity to cracking. The maximum content of 3% corresponds to the content beyond which the elongation and the yield strength become too low for it to remain advantageous to produce parts using an alloy of this type.

The magnesium content lies in the range 0.65% to 1%. This content makes it possible to optimize the density of Mg₂Si precipitates in the aluminum matrix. It compensates for the reduction in the silicon content while also minimizing the macroscopic Mg₂Si precipitates that are damaging and must be dissolved or transformed during the heat treatment. If there are too many precipitates or if they are too big, the heat treatment has only a small effect on their dissolution, since the critical dissolution size is exceeded.

The copper content lies in the range 0.20% to 0.40%. This content makes it possible for Al₂Cu precipitates to be formed in the matrix and for there to be a total absence of macroscopic Al₂Cu precipitates. The absence of any such macroscopic precipitates makes it possible to maintain high forging temperatures and thereby to minimize the forging forces (forging being performed in a single step). The main precipitates formed in the presence of copper are Al₂Cu and AlMgSiCu that melt respectively at 490° C. and at 525° C., and their presence would prevent forging at higher temperatures without running the risk of the alloy being burnt, which would make the parts unusable. Such degradation can be likened to the alloy being destroyed. A higher copper content also increases the sensitivity of the alloy to cracking, because there remains a eutectic to be solidified at low temperatures (490° C. or 525° C.) for which the mechanical stresses (related to shrinkage on solidification) exerted on the part are large.

The manganese content lies in the range 0.15% to 0.25%. This content avoids AlFeSi precipitates forming in β form (highly damaging platelet form) and makes it possible rather for AlFeMnSi precipitates to form in a form (less damaging Chinese-script form). This makes it possible to maximize the elongation on the finished part resulting from the Cobapress method. This effect is most often used with larger quantities of manganese and of iron, since these two elements lead to high hardening of the alloy but also to larger precipitates during solidification. Such large precipitates are detrimental to proper elongation. However, the alloy of the invention is, as indicated, designed for the Cobapress method, in which forging is performed in a single step, and does not involve the large deformations usually encountered in forging, rolling or extrusion. Such large deformations make it possible to fragment the large precipitates and to make them must less damaging, while also maintaining their hardening effect. With the alloy of the invention, the impact of the iron-based precipitates on the mechanical characteristics should be minimized as of the casting stage. This is because their morphology is then no longer modified, since single-step forging does not deform the part sufficiently to change their morphology. Finally, this manganese content is adapted to the cooling speeds obtained when casting in a permanent mold, and, with regard to such speeds, it facilitates the formation of AlFeMnSi precipitates in a form.

The titanium content lies in the range 0.10% to 0.20%. That content is necessary for effective germination of the grains and for a fine grain size that has a large effect on the mechanical characteristics of these alloys.

The strontium content lies in the range 0 ppm to 120 ppm. This content is necessary for having a fibrous solidification of the small quantities of eutectic that are formed. This takes place mainly for silicon contents greater than 1 5%.

It has been seen that the composition of this alloy is adapted to lead to a solidification that makes it possible to maximize the mechanical characteristics in spite of the low levels of deformation encountered during the Cobapress method.

However, solidification defects persist, such as inter-grain shrink-hole solidification defects at the grain joins, with a ramified and diffuse morphology that weakens the casting, i.e. the part resulting from being cast.

The Cobapress forging operation makes it possible to re-close and re-bond such defects with the deformation rate being controlled at the design stage. The temperature/deformation pair makes it possible to solve the defects. The table below gives the mechanical characteristics on a casting and on parts, using the Cobapress method, after T6 heat treatment of the low-silicon alloy. It is possible to note the improvement in the ultimate tensile strength Rm and the ultimate elongation:

STATES Rp_(0.2) [MPa] Rm [MPa] A % [%] Casting AlMgSiCu + T6 300 315 1.3 Cobapress ™ AlMgSiCu + T6 300 340 8 Rp = Yield strength Rm = Ultimate tensile strength A % = Elongation

Finally, this composition makes it possible to reduce the complexity of the usual heat treatment for alloys of the Al—Mg—Si—Cu type. The silicon content, the solidification speeds and the grain refinement lead to macroscopic Mg₂Si precipitates that are of size and of morphology that facilitate dissolution during the heat treatment.

Reference is made to the figures of the accompanying drawings that show the metallographs of a part in order to show the importance of the manganese content and of the copper content. FIG. 1 shows a casting microstructure, without any manganese, and “needle” precipitates of the β type, while FIG. 2 shows the monostructure with manganese, and “Chinese script” precipitates of the a type.

FIGS. 3, 4, and 5 show the elimination of the Al₂Cu copper precipitates.

In FIGS. 3 and 4, the copper content is greater than 0 40%, which leads to the presence of Al₂Cu precipitates. FIG. 4 shows an example in which it is possible to observe the AlFeMnSi and Mg₂Si precipitations surrounded by Al₂Cu precipitates.

FIG. 5 shows a copper content lying in the range 0.20% to 0.40%, in accordance with the invention, showing absence of Al₂Cu precipitates. 

1. A method of obtaining a part made of low-silicon aluminum alloy, containing the alloy comprising: silicon at a content lying in the range of 0.5% to 3%; magnesium at a content lying in the range of 0.65% to 1%; copper at a content lying in the range of 0.20% to 0.40%; manganese at a content lying in the range of 0.15% to 0.25%; titanium at a content lying in the range of 0.10% to 0.20%; and strontium at a content lying in the range of 0 ppm to 120 ppm; said method including: casting said alloy in a mold so as to obtain the part; after the casting, demolding the part constituting a preform that is still hot; cooling said preform and then subjecting said preform to an operation adapted for reheating saisd preform to a temperature lying in the range of 470° C. to 550° C.; positioning said part between two shells of a die that defines a cavity of dimensions substantially equal to but less than the dimensions of the cavity of the mold; and strongly pressing the two shells together to exert on the part disposed between said shells a combined effect of pressing and surface kneading.
 2. An automobile part comprising the Use of a part obtained by the method according to claim
 1. 3. An aviation part comprising the part obtained by the method according to claim
 1. 